Recently, an optical component such as a display of a computer, TV, or plasma display, a liquid crystal display, transparent plastic lens, cover of various meters, and window of an automobile or train is often provided with an antireflection coating as its outermost layer in order to improve visibility. Because of the principle of the antireflection, an antireflection coating should have a low refractive index.
A fluororesin is a material which inherently has a low refractive index and a high alkali resistance, and therefore, it has also been used in displays for antireflection purpose. However, because of its molecular structure, fluororesin is more often used as a rubber material, and its use as a coating agent capable of providing a rigid protective coating having an excellent scratch resistance has been difficult.
With the recent development of hydrolyzable silane compound having a perfluoroalkyl group, various coating agent capable of realizing excellent alkali resistance, water repellency, oil repellency, smudge proof property, antireflection property, and the like have been developed by utilizing the characteristic feature of the resin. However, since the perfluoroalkyl group which realizes such characteristic feature is bulky and inactive, the cured coating suffered from low crosslink density, and as a consequence, the scratch resistance was still insufficient even though the coating was considerably harder than the fluororesin.
In order to improve the scratch resistance, JP-A 2000-119634 (Patent Document 1) proposes a method in which various silane compounds including perfluoroalkyl group-containing silanes and tetra alkoxysilanes are simultaneously hydrolyzed; and JP-A 2000-119634 (Patent Document 1) proposes a material containing a bissilane compound as its main component wherein the bissilane compound contains a perfluoroalkylene group as its spacer. While adhesion of acceptable level has been realized in these attempts, antireflection property has still been insufficient since refractive index has not been fully reduced.
The material which has the lowest refractive index is air, and in order to incorporate such air in the structure of the cured material, use of inorganic fine particles having voids in its interior has been contemplated, and JP-A 7-133105 and JP-A 2001-233611 (Patent Documents 3 and 4) propose porous and hollow silica sols. JP-A 2002-79616 (Patent Document 5) proposes a method in which such material is used by mixing with a silicone containing a fluorine-substituted alkyl group, and JP-A 2004-272197 (Patent Document 6) attempts a method in which such material is used by dispersing in a binder component containing an ionization radiation-curable monomer. When the inventors of the present invention confirmed such method, the hollow silica sol could be homogeneously mixed with the solution of such binder in an organic solvent. However, after the volatilization of the organic solvent, floating of the hollow silica sol containing voids in its interior toward the surface of the coating was observed. As a consequence, despite the favorable antireflection property of the entire cured coating, the cured coating did not have sufficient scratch resistance since a large amount of hollow silica particles having hollow structure and inferior strength were present on the surface of the coating, and also, immobilization of such particles by the binder component was insufficient. The coating was also found to have insufficient alkali resistance due to the inferior alkali resistance of the silica component.
In order to obviate the drawbacks of the hollow silica sol, JP-A 2005-266051 (Patent Document 7) attempts use of a hollow silica sol which has been surface treated with a silane compound containing a fluorine-substituted alkyl group by mixing and dispersing the surface treated hollow silica sol in a binder derived from a tetraalkoxysilane. In this case, the alkali resistance has been improved since the surface has been covered with a hydrophobic group. The scratch resistance, however, is not improved because the hollow silica particles are merely mixed with the binder and floating of the hollow silica particles has not been prevented. JP-A 2005-99778 (Patent Document 8) discloses an attempt to disperse a hollow silica sol which has been surface treated with a silane compound containing an ionizing radiation-curable group in an ionizing radiation-curable resin. In this case, while the binder becomes bonded to the particles upon curing, the problem of floating before the curing has not been prevented and the scratch resistance has still been insufficient.
As described above, there has been no coating agent which has realized high level scratch resistance simultaneously with the antireflection property.